The invention relates to a pressure wave sensor consisting of a piezo electric element secured in a cylindrical housing where the piezo electric element is pressed by a spring against a front surface of the housing, which front surface comprises at least two layers.
With laser-induced photoacoustic spectroscopy, a transparent sample including the compound to be subjected to spectroscopy (for example, an aqueous solution) is irradiated by short repetitive light impulses of high intensity from a pulsed laser light source. Because of the photothermal absorption and the resulting local heat expansion, shock waves are generated in the sample. A pressure wave sensor (piezo ceramic detector) which is firmly engaged with the sample converts the pressure waves into a voltage signal. The time-dependent voltage curve ideally corresponds to an attenuated oscillation and is proportional to the absorption in the sample.
The following equation applies:
Piezo voltage U=k.multidot.(.beta..multidot.v.sup.2 /c.sub.p).multidot..alpha..multidot.I.sub.o,
wherein:
.beta.=.beta.(T): thermal expansion coefficient of the sample,
v=v(T): sound velocity in the sample,
c.sub.p =c.sub.p (T): specific heat at constant pressure,
.alpha.=.epsilon..multidot.c: absorption coefficient
.epsilon.: molar extraction coefficient
c: concentration
I.sub.o : laser energy
For the recording of absorption spectra, the wave length of a narrow band excitation laser (for example Nd: YAG--pumped coloring agent laser) is tuned and recorded with the respective piezo voltage. With an appropriate experimental arrangement detection limits are reached (signal/noise ratio S/N=3) which are lower than those common in the conventional high-resolution absorption spectroscopy by about 2 orders of magnitude.
Such a pressure wave sensor is known for example, from J. I. Kim, R. Stump, R. Klenze, "Laser-induced Photoacoustic Spectroscopy for the Specification of Transwavic Elements in Natural Aquatic Systems", Topics in Current Chemistry, Vol. 137, Springer Verlag Berlin, Heidelberg, 1990, p. 131-179.
On pages 148-155, various photoacoustic measuring cells are described. In these measuring cells, the piezo detector is disposed in metal housings to be shielded from electromagnetic radiation, the housings being coupled to quartz glass cuvettes for the transmission of sound waves. The coupling is generally achieved by spiral springs which are supported on the piezo housing or respectively, on corresponding support structures or by the weight of the sample to be measured. (The sample is disposed in the PZT detector). A gel-like liquid introduced between the detector housing and the sample, or respectively, between the PZT detector and the detector housing, improves the sound wave transmission.
Since the vibration systems (piezo detector, housing, engagement springs) are not optimized with regard to the properties referred to earlier (maximum voltage signal, vibration only at the piezo detectors frequency, no harmonic vibration), it has not been possible to utilize the maximally possible signal amplitude. furthermore, the interpretation of the detected piezo signals is difficult, particularly at low sample concentrations.
With the arrangements described reproducible measurement signals were difficult to obtain because of the geometric arrangement (for example, the sample cuvette disposed on the detector) or because of the use of additional contacting means for the sound transmission. Furthermore, the use of contacting means requires additional cleaning steps for the sample cuvettes which have to comply with the highest optical and chemical purity requirements.
For a functional 2-channel operation, that is, for on-line underground subtraction (subtraction of the LPAS-signal of the solvent from the LPAS-signal of the cuvette), it is necessary that both piezo detectors provide identical signals when they are excited in the same manner. In the state of the art, this was possible only in a limited way since encapsulated detectors providing identical signals could not be made.
With the limitations in the use of the laser-induced photoacoustic spectroscopy, highly sensitive optical measurement procedures are not routinely available. Although this method is substantially more sensitive than the conventional transmission spectroscopy, there are only very few apparatus of this type in existence.
It is the object of the present invention to provide a pressure wave sensor of the type referred to earlier, which however has an improved detection sensitivity.